Regarding vaporized fuel treatment mechanisms for preventing fuel from being discharged into the atmosphere. On Board Diagnosis (OBD) guidelines established by the State of California provide that all North American vehicles manufactured after 1994 should be fitted with a system for testing for faults in a vaporized fuel treatment mechanism. These guidelines stipulate that when there is a leak hole of more than 1 mm diameter in a flowpath from a fuel tank to a purge cut valve, the leak must be detected, and a warning lamp lighted.
A diagnostic system in a vaporized fuel treatment mechanism meeting these requirements is disclosed for example in U.S. Pat. No. 5,542,397.
This system comprises a drain cut valve in a fresh air inlet port of a canister to make the flowpath a closed space, and a pressure sensor inserted in the flowpath. After the closed space has been converted to low pressure using intake negative pressure of the engine, the cross-sectional area of the leak hole is calculated based on the variation of flowpath pressure detected by the pressure sensor.
When the fuel in the fuel tank sloshes around or the liquid surface in the tank vibrates due to for example travel of the vehicle on a winding road, the amount of vaporized fuel in the tank sharply increases and the pressure in the flowpath rises. This phenomenon will be referred to as sloshing in the following description. If leak testing is performed under such a condition, it is possible that the result of the test will be erroneous. Referring to FIG. 4A of the drawings, according to the diagnostic algorithm of this device, the flowpath pressure and an elapsed time DT.sub.4 are sampled at a point B at which the flowpath pressure has risen by a predetermined amount p.sub.3 above its value at a point A. However, when a pressure change occurs due to sloshing as shown by the broken line of the figure, the flowpath pressure and elapsed time DT.sub.4 are sampled at a point C. An error therefore occurs in the calculation of leak hole surface area, and consequently, the leak hole surface area corresponding to a time difference between the point B and point C is added to the real leak hole surface area.
To address this problem, Tokkai Hei 6-159157 published by the Japanese Patent Office in 1994 compares a variation amount .DELTA.P in a predetermined interval of flowpath pressure with a predetermined value .alpha., determines that sloshing has occurred when .DELTA.P is equal to or greater than .alpha., and stops leak testing at that time.
However, when the determining level .alpha. for determining sloshing is a fixed value, sufficiently high precision of the leak test is not obtained. FIG. 4B shows a variation amount .DELTA.EVPRES per predetermined interval of flowpath pressure in FIG. 4A. In this case, sloshing 1 shown by the broken line in the figure is correctly determined. However, as .DELTA.EVPRES is equal to or greater than .alpha. before a point D, it is incorrectly determined that sloshing has occurred regardless of whether or not it really did occur. To avoid such an incorrect determination, the sloshing determination must therefore be performed only after the point D, and as a result, sloshing 2 prior to point D cannot be detected.